Fiber optic frequency shifters are useful devices for a variety of fiber optic sensor and signal processing applications, such as in fiber optic gyros and the like. It has been shown, for example in copending U.S. patent application Ser. No. 556,636 now U.S. Pat. No. 4,684,215, "Single Mode Fiber Optic Single Side Band Modulator", filed on Nov. 30, 1983, and assigned to the assignee of the present application, that light launched in one propagation mode of a fiber can be coupled to another propagation mode and shifted in frequency by propagating an acoustic wave along the length of the fiber to cause a periodic stress of the fiber. The light is shifted in frequency by an amount equal to the frequency of the acoustic wave. The copending U.S. patent application Ser. No. 556,636 U.S. Pat. No. 4,684,215 is incorporated herein by reference.
As set forth in the above-referenced copending application, if the acoustic wave propagates along the fiber in the same direction as an optical signal propagates through the fiber, light traveling in a first propagation mode in the fiber at a first phase velocity and at a first frequency is coupled to a second propagation mode at a second phase velocity higher than the first phase velocity and is shifted downward in frequency. Similarly, if the light is originally propagating in the fiber in a faster propagation mode, the light is coupled to a slower propagation mode at a higher frequency. On the other hand, if the acoustic wave propagates along the fiber in a direction opposite the direction that an optical signal is propagating through the fiber, light traveling in a slower propagation mode is coupled to a faster propagation mode and is shifted upward in frequency. Similarly, light traveling in a faster propagation mode opposite the direction of propagation of an acoustic wave is coupled to a slower propagation mode and is shifted downward in frequency.
For optimal coupling to occur in the frequency shifter described in the above-referenced copending application, the acoustic wavelengths in the direction of propagation of the optical signal through the fiber should be substantially equal to the beat length of an optical signal traveling through the fiber. As is well known, when light travels through a fiber in more than one propagation mode, the light travels through the fiber at a different phase velocity for each of the different propagation modes. Light traveling in the slower propagation mode travels at a lower phase velocity than light in a faster propagation mode. Thus, a light signal having a fixed frequency will have a longer wavelength in the faster propagation mode than it has in the slower propagation mode. As the light propagates down the length of the fiber, a phase difference will thus develop between the light in the two modes. At spatially periodic distances, the light in the two modes will be in phase. The distance between successive locations where the light is in phase is referred to as the beat length of the fiber for the two modes at a specified frequency.
For the devices discussed in the above-mentioned co-pending application, this is the beat length that must match the acoustic wavelength in order to achieve optical coupling of energy, between the modes. The light propagating along a fiber in one propagation mode is converted to light propagating in a second propagation mode by applying a periodic, traveling wave, compressive force along a segment of the length of the fibers. A more complete description of this technique is found in "Single-Sideband Frequency Shifting in Birefringent Optical Fiber", W. P. Risk, et al SPIE Volume 478--Fiber Optic and Laser Sensors II (1984), pp. 91-97, which is incorporated herein by reference.